The invention relates to a method of applying optical coatings of silicon compounds to substrates by reactive cathode sputtering of siliceous target materials.
It is known to use, for the production of coatings of silicon compounds, a planar target material which has been formed under high pressure from silicon powder containing additional materials such as aluminum and lithium to produce electrical conductivity and improve thermal conductivity. For the achievement of a high density of the target material, the compression is performed at high temperatures on the order of 2000.degree. C. Due to this complex manufacturing process such targets are very expensive. For example, a target plate measuring 70 cm.times.25 cm.times.1 cm costs between DM 25,000.00 and DM 40,000.00. At the same time, hot-pressed target material has the disadvantage that, due to its still-grainy or porous texture, it has a large surface area, and this necessitates a relative long period of time for the process of cleaning the target material for each new batch. This means a preliminary sputtering of the target surface onto a plate or diaphragm temporarily inserted between the target and substrate. The correspondingly long time required for the cleaning process results in a corresponding loss of material, but also makes the sputtering apparatus less economical to operate, since the actual coating process does not begin until the cleaning process is completed.
Moreover, the surface of a pressed material is rough, from the microscopic viewpoint, and as the sputtering process advances so-called bumps or humps can form on the surface. This is to be attributed basically to the different rates of sputtering of the components contained in the target material, such as aluminum and lithium, for example. On account of the low thermal conductivity between the humps and the rest of the material a local overheating occurs which causes solid particles of the target material to burst out of the target plate. Even if these particles do not come in contact with the substrates, however, this process has a negative effect on the growth and homogeneity of the layer deposited on the substrate.
Reactive cathode sputtering is chiefly a process of oxidation that occurs while the coating is in progress, i.e., oxygen is contained in the reaction gas, and oxides of silicon, mostly silicon dioxide, are formed. Similar considerations apply, of course, to other reaction gases, nitrogen for example, if silicon nitride is to be formed on the substrates.
The above-described reactive processes are not free, either, of adverse effects on the target materials. In the case of hot-pressed target materials, there is especially the tendency to form on the sputtering surface layers of the product of the reaction with the reaction gas, that is, layers of silicon dioxide in the case of oxygen. Such reaction products are electrical nonconductors, so that the sputtering process is at least locally hampered, or in any case is impeded unless a high frequency is used for the sputtering process. On account of the greater simplicity of the technique, however, there is a tendency to use so-called diode sputtering insofar as possible, along with a magnetic field enhancement of the sputtering process in some cases, in order to raise the otherwise low sputtering rate by at least one power of ten. The classical hot-pressed target material, however, leads to the problems stated above.
The article by Thornton and Hoffmann, "Internal Stresses in Amorphous Silicon Films Deposited by Cylindrical Magnetron Sputtering using Ne, Ar, Kr, Xe and Ar +H.sub.2 ", which appeared in J. Vac. Sci. Technol., 18 (2), Mar. 1981, pages 203 to 207, discloses the use of polycrystalline silicon, drawn in rod form from fusion, as a cathode in a sputtering process wherein the cathode is provided with doping to achieve the necessary electrical conductivity, and the doping material has to be diffused into the cathode material by an exceedingly tedious process. The described cathode sputtering process is not part of a production process, but serves only for the study of internal tensions in the coatings thus produced. The production of the rod-shaped cathode calls for the insertion of a narrow slit diaphragm, so that a very large part of the material sputtered away from the cathode is deposited on the diaphragm, not on the substrate. In any event, this state of the art has to do with the production of coatings having special electrical properties, not coatings for optical applications.
The article by Severin, entitled, "Materialien fuer die Kathoden-Zerstaeubung," which appeared in Vakuum-Technik, vol. 33, No. 1, pp. 3 to 9, discloses the production of cast targets made from an aluminum-silicon alloy. The aluminum content, however, is very high, since the aluminum is intended to produce not only a good electrical conductivity permitting sputtering by direct current, but also to permit plastic mechanical deformation for the purpose of subsequent "grain fining." With such targets, however, homogeneous coatings of virtually pure silicon dioxide cannot be produced, since at least aluminum oxide is an important component of the coatings, and silicon and aluminum behave differently in the sputtering, so that the relative proportions of the oxides change as the consumption of the target progresses.
Optical coatings of silicon compounds, especially those of silicon dioxide, however, are playing a vital role ino today's coating technology, both as a final protective coating on large-area substrates such as windowpanes, for example, and as a material for multiple coatings built us of alternating high and low refraction layers (known as "interference filters").
It is therefore the object of the invention to device a method of the kind described above, in which the utilization of the target material, consisting of at least 99% silicon, will be improved and the danger of particles bursting out of the target material will be eliminated or at least greatly reduced.